Heretofore, a glass capillary tube having an inner diameter of at least about 1 mm and an outer diameter of about 1 cm useful for a thermometer or for transportation of a fluid, has been produced directly from molten glass. A fine glass capillary tube having an inner diameter of from 0.1 to 0.2 mm useful as a ferrule for optical fibers in recent years, has been produced by hot stretching the glass capillary tube directly produced from molten glass as mentioned above.
However, in the method for producing the capillary tube directly from molten glass, it is extremely difficult to produce stably with high precision a capillary tube having an inner diameter of less than 0.5 mm or an outer diameter being at least about 10 times the inner diameter. In the case of producing a finer capillary tube, the capillary tube produced directly from molten glass as mentioned above is used as the starting preform, whereby it is extremely difficult to bring the inner diameter to a level of about 1/10 of the outer diameter.